Greenhouse Weed Management: A Critical Step in Minimizing Pests and Disease

As a young grower working in a small greenhouse operation in the late 1970's I can remember the carpet of weeds that grew profusely under our greenhouse benches. The oxalis or woodsorrel was so thick that on quiet days with no fans blowing, you could hear the faint sound of the seed striking the greenhouse plastic as their seed capsules ripened and ruptured. Greenhouse whitefly thrived in the verdant weed growth under these benches and any insecticide applications made to the plants growing above had to be retargeted to kill the pests that sought harborage in the weeds below.

Weed management in the greenhouse is very tough because there are very few chemistries that are currently labeled for use in the greenhouse environment. Hand weeding is a viable option to reduce weed populations, but no employee will relish the task of crawling under greenhouse benches to remove weeds.

Pre-emergent herbicides prevent weed seeds from germinating or kill the roots of the germinating seed prior to shoot emergence from the soil. Post-emergent herbicides are used to target weeds/plants after germination and after plant tissues have emerged from the soil.

One pre-emergent herbicide that greenhouse growers can utilize is the product Marengo from OHP. Indaziflam which is the active ingredient in Marengo is considered to be a residual pre-emergent herbicide. It cannot be used in a greenhouse where crops are present and it has a 12-hour REI. Marengo has shown excellent activity on a range of perennial and annual weeds. Some of the weeds considered to be suppressed or controlled in the greenhouse environment by Marengo include bittercress, chickweed, shepherd's purse, woodsorrel, crabgrass, and foxtail.

Marengo as per the OHP label can be tank-mixed with the post-emergent herbicide, glufosinate (Finale) to provide both pre and post-emergent control of weeds in the greenhouse. Marengo used alone or in a tank-mix should only be applied between cropping cycles when the greenhouse is empty to comply both with the legal label and to prevent plant injury.

Post-emergent herbicides labeled for use in greenhouses can either be selective for grasses or non-selective in their approach. Most broadspectrum non-selective herbicides are membrane disrupters and can be used with caution to control emerged weeds in the greenhouse.

Glufosinate (Finale) is a foliar-applied, non-selective, contact herbicide that has no soil activity. Glufosinate inhibits glutamine synthetase which prevents the conversion of glutamate and ammonia to glutamine. While not considered a cell-membrane disrupter it is used as a burn-down material in many cropping systems.

Finale,a Bayer product containing glufosinate is labeled for use in shadehouses and greenhouses. When using it in a greenhouse all circulation fans should be shut-off during application to prevent spray drift from contacting desirable plants/flowers. Finale is to be only used as a directed spray using large droplet, low-pressure type nozzles. Finale should not be used in greenhouses or shadehouses where edible crops are being grown.

If edible crops are being grown cell membrane disrupters may be the most effective class of herbicides to deploy in protective culture systems (greenhouses). Pelargonic acid (Scythe) is distributed/marketed by Gowan. It can be legally applied in greenhouses as a directed spray using low-pressure and large droplet type nozzles to limit the likelihood of spray drift. As an added precaution, all circulation fans should be shut-off to prevent smaller spray droplets from drifting into the crop canopy. Pelargonic acid (Scythe) has a 12 hour REI.

Ammonium nonanoate (Axxe) marketed by Bio-Safe Systems is another contact herbicide that is labeled for post-emergent control of weeds in greenhouse structures. It has no soil activity and functions as a burndown material. Axxe can be used in the greenhouse when crops are present provided that the circulation fans are turned off and that the herbicide is applied using low pressure and a coarse droplet size. Axxe requires a 24 hour REI and can be used in greenhouses where edible crops are being grown.

Lemongrass oil (GreenMatchEX) which is marketed by Marrone Bio-Innovations is a post-emergent burndown herbicide that is labeled for use in the greenhouse. It has a 0-hour REI, but workers should not re-enter a treated area until the product has dried. Lemongrass oil should be directly applied to weeds using low pressure and coarse spray droplets. Air circulation fans should be shut-off to prevent pesticide drift. GreenMatch EX is OMRI approved and can be used where organic and edible crops are being grown.